The present invention relates generally to communication networks, and more specifically, to a method and system for handling recursive BGP (Border Gateway Protocol) route updates in MPLS (MultiProtocol Label Switching) networks.
The rapid growth of the Internet and the widespread deployment of networks built around the Internet Protocol suite are creating a demand for new capabilities in IP (Internet Protocol) networks. MPLS provides a number of powerful capabilities such as traffic engineering, etc. As with IP routers, MPLS nodes use a routing protocol such as OSPF or IS-IS to calculate network paths and establish reachability. Multiprotocol Label Switching is an IETF initiative that integrates Layer 2 information about network links (bandwidth, latency, utilization) into Layer 3 (IP) within a particular autonomous system in order to simplify and improve IP packet exchange. MPLS provides network operators a great deal of flexibility to divert and route traffic around link failures, congestion, and bottlenecks. MPLS based networks are becoming increasingly important for today's backbone Internet.
In MPLS, data transmission occurs on label-switched paths (LSPs). LSPs are a sequence of labels at each and every node along the path from the source to the destination. LSPs are established prior to data transmission (control-driven). The labels, which are underlying protocol-specific identifiers, are distributed using label distribution protocol (LDP) or RSVP or multiprotocol border gateway protocol (MPBGP). Each data packet encapsulates and carries the labels during their journey from source to destination. High-speed switching of data is possible because the fixed-length labels are inserted at the very beginning of the packet or cell and can be used by hardware to switch packets quickly between links.
With MPLS, incoming packets (predominantly IP packets) are assigned with a label when the packets enter the MPLS network. Packet forwarding is done based on label information within the MPLS core network. In today's routers, packets are typically forwarded with the assistance of ASIC hardware. After an IP packet enters a router, the router decides the next hop or path the packet needs to go by looking up the routing table, typically using the destination address of the IP packet.
MPLS enabled networks are becoming increasingly important for today's service providers (SPs) in the design and deployment of current and future networks. A primary contributing factor is that MPLS, as an enabling technology, has the capability of converging not only voice, data, and video, but also frame, cell, and packet networks into a single network. Deploying and managing a single and scaleable network is a great benefit to service providers. Furthermore, MPLS networks also allow service providers to do traffic engineering, and quickly reroute customer traffic upon identification of a link failure within their network. This is an important feature for carrier class networks.
One of the key advantages that MPLS provides is that the vast amount of BGP routes that must be available in every router in the SP network is only needed at the provider edge router (PE). This is accomplished by the combination of BGP (MP-BGP for MPLS/VPN) and LDP (Label Distribution Protocol) running in the core. BGP is a routing protocol that can be used to exchange routing information between different or same autonomous systems. Customer networks usually use an Interior Gateway Protocol (IGP) such as RIP or OSPF for the exchange of routing information within their networks. Customers connect to ISPs and ISPs use BGP to exchange customer and ISP routes. BGP neighbors exchange full routing information when the TCP connection between neighbors is first established. When changes to the routing table are detected, the BGP routers send to their neighbors only those routes that have changed. BGP routers do not send periodic routing updates, and BGP routing updates advertise only the optimal path to a destination network.
Routers which have been configured to serve the SP network must be capable of switching data traffic at very high rates and processing a large amount of routing information very efficiently. Routers are often expected to recover from a link failure in a very short period of time (e.g., 50 msec) in order to serve the carrier network space. Fast ReRoute is one application of MPLS Traffic Engineering which enables customers to set up a backup network path with certain bandwidth requirements and explicitly route traffic down that path in the event of an interface or link failure.
Currently, high speed data switching is typically accomplished with the assistance of ASIC based switching engines. When an IP packet enters the PE router, the switching engine looks up its routing table, which may contain several hundred thousand routes. For MPLS/VPN case, the switching engine then pushes two labels (a top label and a bottom label) on top of the IP packet. The top label is an IGP (Interior Gateway Protocol) label for routing the packet across the network core. The bottom label is a VPN label for routing the packet to its final destination when it enters an egress PE router.
When a customer's IP packet enters the ingress PE of a SP's MPLS/VPN network, the PE's forwarding engine needs to lookup its routing table (using the packet's IP destination address). If there is a match, the packet will carry two labels when it leaves the ingress PE and enters the MPLS core network. Forwarding in ASIC for this scenario typically involves a hardware assisted lookup using the packet's IP destination address. If there is a match, the lookup result provides the forwarding engine with the location of a hardware adjacency entry, where the two labels and outgoing interface information are stored, to rewrite the packets with the correct label information before sending out the packet.
While traversing the core network to reach its final destination, the packet's IP destination address is no longer used. Instead the top (IGP) label is used to transmit the packet to the egress PE and the bottom (BGP/VPN) label is used for the egress PE to get to the final destination within the customer network. For example, if there are 500,000 different BGP/VPN routes, there will be 500,000 different BGP/VPN labels while there are only a few routes (if load sharing paths are considered) for the ingress PE to get the customer packets to the egress PE.
The conventional approach for the ingress PE to forward the packet is as follows. Each of the customer routes (BGP/VPN) is programmed in its ASIC FIB (Forwarding Information Base) table. Each route is then associated with an ASIC adjacency entry, where the IGP label, BGP/VPN label, and source/destination MAC are rewritten to the outgoing packet. Their relationship and dependency are summarized in FIG. 1. Prefixes 100.1.1.0/24, 101.1.2.0/24, etc. are BGP routes. There are typically several hundred thousand routes. Labels 10001 and 10002 are BGP/VPN labels. Labels 100 and 200 are IGP labels. In this example, there are two loadsharing paths for each BGP route, represented by label 100 and 200, respectively.
The conventional approach described above works well for switching packets very fast, even when a large size routing table is present. However, it does not scale when the link between the PE and one of its immediate provider routers fails. This is because it takes a considerable amount of time to reprogram the ASIC switching engine to use a new IGP label to get to an alternative provider router for each of the BGP routes in it its routing table. The size of the BGP table may contain as many as several hundred thousand routes. Thus, latency in updating the ASIC forwarding engine will typically result in a long disruption of customer traffic.
There is, therefore, a need for a scaleable scheme for a forwarding engine to handle, for example, when a link between a provider edge router and its neighboring provider routers goes down or up and only the IGP routes for the network core (typically a small number of the BGP routes) need to be reprogrammed in the forwarding engine.